The range of the use of a cerium-based abrasive containing cerium-based particles as a main component has rapidly been increasing owing to the excellent polishing effects and the functions of a variety of additives added to the abrasive. Today, it is employed not only for a conventional purpose of polishing optical glass but for the fields of polishing glass for a liquid crystal, glass for a magnetic recording medium such as hard disks, and fabricating an electronic circuit such as LSI.
The cerium-based abrasive has generally been produced by the following method. That is, raw materials are made to be a slurry, wet-pulverized, treated with a mineral acid as required, and then subjected to chemical treatment with hydrofluoric acid, ammonium fluoride, and the like. The obtained slurry is filtered, dried, and roasted, the resulting particle is then pulverized and classified to obtain an abrasive particle having a desired particle diameter. Rare earth raw materials such as rare earth carbonates, rare earth hydroxides, rare earth oxalates, or rare earth oxides obtained by firing these compounds are used as the raw materials of the cerium-based abrasive.
These rare earth raw materials have been produced generally by removing some rare earth metals (Nd, Pr, etc.) and radioactive elements from bastnasite-based rare earth raw materials or cerium-containing rare earth raw materials by a well-known chemical treatment.
The cerium-based abrasive is assumed to have a high polishing speed and further is required to produce a polished face with excellent specular properties. Although these properties are required not only for cerium-based one but also for an abrasive generally, the cerium-based one is required to be excellent in the polishing properties and to be improved in other properties as well.
For example, the cerium-based abrasive is generally dispersed in a dispersion medium such as water and mixed with a dispersant, a pH adjusting agent, and the like, as required, and then supplied to a polishing apparatus. Consequently, it is important for a dried abrasive particle powder to be quickly dispersed in water and to become an even slurry.
Further, the adhesive property of the abrasive particles to the surface of an object to be polished is also important. This is because the object to be polished is to be washed after polishing and if the adhesive property of the abrasive particles is high, the abrasive particles remain on the surface of the object to be polished after washing. Hence, the remaining abrasive particles may sometimes become causes of scratches in handling thereafter. Further, if the object to be polished is a substrate for a hard disk, the existence of the remaining particles becomes causes of deterioration of the smoothness in the case where a magnetic material is vaporized and deposited after polishing.
As the measures to satisfy such various requirements, in terms of the improvement of the polishing properties, in addition to improvements of an abrasive, various amendments of additives to a slurry, an abrasive pad, a polishing apparatus and the like have been performed. However, in terms of the improvement of the dispersibility and the adhesive property, in many cases, additives have been employed to meet such a requirement based on their functions, yet any satisfactory countermeasures to satisfy the requirement for the improvement of the abrasive particles themselves have not been proposed and no abrasive particle excellent both in the polishing properties and the dispersibility has been proposed. Especially, regarding the cerium-based abrasive, abrasive particles with an extremely small particle diameter of 1 μm or smaller, further 0.5 μm or smaller, have been employed in order to realize highly excellent surface smoothness and specular property, whereas it is supposed that the dispersibility and the adhesive property are deteriorated in relation to the surface energy if the particle diameter of particles becomes fine. However, no countermeasures against it have not yet been performed.
The invention has been developed under the above described situation and aims to provide a cerium-based abrasive excellent in the polishing capability and the polishing precision and also excellent in the dispersibility and other properties.